Kustom K100-1 bad channel 2

[xtian]

Please consider, I'm a valve amp guy, with far less knowledge of SS amps, so these are very basic questions. Channel 1 works fine (top channel in schematic) but channel 2 doesn't pass signal. Voltage survey for first three transistors:



I think this shows Q108, Q109, Q110 are not conducting properly and should be replaced. True, or could it be supporting components at fault?

Advice appreciated.

[The Dude]

I would do a quick check of transistor junctions in circuit to see if you spot anything amiss. If one checks bad in circuit, pull it and check out of circuit. Q109 voltages may indicate a bad transistor. If working correctly and biased on, the collector should be pulled more positive. It looks to be not conducting.

[xtian]

I would do a quick check of transistor junctions in circuit

Please explain. All I know how to do is look for shorts by measuring resistance. These transistors don't show any shorts.

I also have a cheap tester: MK-168 Transistor Tester...
Q100 reads: NPN, ICEs=0.08mA, B=54, Uf=831mV
Q108 reads: NPN, ICEs=0.09mA, B= 8, Uf= 239mA

[The Dude]

A quick basic transistor test (and it doesn't tell you everything), is to check base-emitter and base-collector using diode check. Each junction should test like a diode dropping approximately .6V one direction and reading open the other direction. Observe polarity depending on PNP or NPN transistor. Then check collector to emitter and it should read open on diode check. Again, that is just a quick cursory test. if something reads radically wrong, then test out of circuit because other circuit components can effect readings. Your voltages above indicate that Q109 is not conducting, so I'd bet the transistor is open and not shorted. If you Google something like "How to test a transistor with a DVM", you'll find lots of tutorials. I hope that is helpful.

[xtian]

I replaced Q108 with a 2N5088, and that solved it. All working good now. Thanks for the help!

[The Dude]

Not sure I was much help. I speculated the wrong transistor. Glad you got it solved!

[loudthud]

These things have a tendency to blow up input transistors. Best to install a 3 wire Mains cable and remove the Death cap.

[xtian]

These things have a tendency to blow up input transistors. Best to install a 3 wire Mains cable and remove the Death cap.

Did that. Thanks!

[g1]

I also have a cheap tester: MK-168 Transistor Tester...
Q100 reads: NPN, ICEs=0.08mA, B=54, Uf=831mV
Q108 reads: NPN, ICEs=0.09mA, B= 8, Uf= 239mA

I replaced Q108 with a 2N5088, and that solved it. All working good now.

Uf is actually Vf, which is the forward voltage drop. This is like the number that the diode range on a multimeter would give you. So even though Q108 did not show as short on the resistance range, it would have showed as bad. ( too low a V drop, .239).
Good silicon junctions are normally in the .4 to .7 range on a diode check function of a meter.

It's possible your MK-168 needs to run through it's calibration routine. This page tells how to do it, as well as a rundown on what some of the numbers mean:
https://chinese-electronics-products...er-tested.html

[Helmholtz]

In Europe the traditional/standardized symbol for voltage is U (not V). V is used for electrical potential, so Vx-Vy = Uxy.

Also the low beta of 8 indicates a bad transistor.

[g1]

In Europe the traditional/standardized symbol for voltage is U (not V). V is used for electrical potential, so Vx-Vy = Uxy.

I don't understand this statement. Is electrical potential not voltage? How can the difference between 2 voltages (x and y) be something other than voltage?
I can accept that Europe uses U instead of V as terminology, but I don't get how they would be two different things?

[Helmholtz]

I don't understand this statement. Is electrical potential not voltage? How can the difference between 2 voltages (x and y) be something other than voltage?
I can accept that Europe uses U instead of V as terminology, but I don't get how they would be two different things?

Any point in space has electric potential. Voltage is defined as the potential difference between 2 points (it follows that both share the same unit Volt).
So if 2 points have the potentials 1000V and 1200V, the voltage between is 200V.

The voltage of a single point is not defined, doesn't make sense and couldn't be measured, that's why voltmeters need 2 leads.

So it makes sense to use different symbols. In US literature I've see the greek letter phi used for potentials.

https://en.wikipedia.org/wiki/Voltage
https://en.wikipedia.org/wiki/Electric_potential

[g1]

Any point in space has electric potential. Voltage is defined as the potential difference between 2 points (it follows that both share the same unit Volt).
So if 2 points have the potentials 1000V and 1200V, the voltage between is 200V.

The voltage of a single point is not defined, doesn't make sense and couldn't be measured, that's why voltmeters need 2 leads.

So it makes sense to use different symbols. In US literature I've see the greek letter phi used for potentials.

Ok, I think I get it. Your formula above "Vx-Vy = Uxy" is using the European V, the formula for north america would have the phi instead. I was reading it as both the V and U being voltage, so it didn't make much sense in that context.
Trying to grasp the idea of potential with no point of reference is difficult for me.

[Helmholtz]

Trying to grasp the idea of potential with no point of reference is difficult for me.

It surely makes sense in physics.
But with most technical problems, absolute potential and point of zero potential are not relevant.
Means the point of zero potential can be freely chosen.
So you're free to define your chassis ground to be at 0V (potential not voltage!). This is called a gauge transformation in physics.